The present invention relates to charged particle accelerators and, more particularly, to methods and apparatus for operating high energy accelerators in a low energy mode.
Ion implantation is a standard, commercially accepted technique for introducing conductivity-altering impurities into semiconductor wafers. In a conventional ion implantation system, a desired impurity material is ionized in an ion source, the ions are accelerated to form an ion beam of prescribed energy, and the ion beam is directed at the surface of the wafer. The energetic ions in the beam penetrate into the bulk of the semiconductor material and are embedded into the crystalline lattice of the semiconductor material to form a region of desired conductivity.
Exacting requirements are placed on semiconductor fabrication processes involving ion implantation with respect to the cumulative ion dose implanted into the wafer, implant depth, dose uniformity across the wafer surface, surface damage and undesirable contamination. The implanted dose and depth determine the electrical activity of the implanted region, while dose uniformity is required to ensure that all devices on the semiconductor wafer have operating characteristics within specified limits.
To form devices on the semiconductor wafer, it is usually necessary to implant impurities at different depths. The energy of the particles in the beam is determinative of the depth to which the particles penetrate into the semiconductor wafer. As devices are reduced in size and increased in speed, it has become desirable to use very low energy beams to form, for example, shallow transistor junctions in the semiconductor wafer.
Ion implantation with a low energy ion beam is not a trivial task, however. The ions within the beam are typically positively charged particles. Electrostatic repulsion of the charged particles causes the beam to diverge, particularly at low energies where the low velocities of the individual particles dictate that the particles remain within the beam for a comparatively longer period of time before reaching the target wafer.
Since a given xe2x80x9crecipexe2x80x9d for fabricating an electronic device on a semiconductor wafer may call for implantation steps at both high and low energies, it may be desirable to control the ion implanter to implant ions over a wide range of implant energies. This avoids the time, additional cost and potential wafer contamination associated with ion implantation in different ion implanters adapted for different energy ranges.
A high energy ion implanter may employ a so-called tandem accelerator which receives a low energy ion beam with energy on the order of a few tens of keV (thousand electron volts) and further accelerates the ion beam to energies in the range of several hundred to several thousand keV. A tandem accelerator typically includes a low energy accelerator tube, a terminal, and a high energy accelerator tube assembled to form an in-line structure known as an accelerator column. The accelerator tubes contain a number of accelerator electrodes separated by insulating rings. A high positive voltage is applied by a high voltage supply to the terminal and thereby to the highest voltage electrodes of the low energy and high energy accelerator tubes. Adjacent accelerator electrodes are interconnected by high value resistors which distribute the applied voltage among the accelerator electrodes. The terminal between the first and second accelerator tubes contains a gas-filled stripper tube which converts ions in the beam from a negative charge to a positive charge. In a normal high energy mode, a negative ion beam is injected into the tandem accelerator, is accelerated through the low energy accelerator tube to the terminal, is converted to a positive beam and then is accelerated further in the high energy accelerator tube.
To produce beams at low energy, it is desirable to inject a positive ion beam into the tandem accelerator and to turn the high voltage power supply off. However, stray potentials may remain on the accelerator electrodes after the high energy accelerator is deactivated. Furthermore, the fringes of the low energy ion beam may strike the accelerator electrodes and cause the electrodes to develop a positive voltage. The resistors connected between accelerator electrodes, typically on the order of 100 megohms, are insufficient to discharge the electrodes during low energy operation. The accelerator electrodes are located in a high voltage tank that is pressurized with SF6 gas and are not accessible during operation. The result is that the positive voltages on the accelerator electrodes may remove free electrons from the ion beam during low energy operation. Electrons that travel with the positive ions in the ion beam have the beneficial effect of reducing the tendency for space charge expansion of the ion beam. Thus, positive voltages on the accelerator electrodes during low energy operation exacerbate space charge expansion of the ion beam and reduce the beam current transported through the accelerator.
Accordingly, there is a need for methods and apparatus for operating high energy accelerators in a low energy mode.
The present invention provides an ion implanter capable of operating both at high energy and at low energy and enables efficient low energy operation. A switch assembly is configured to connect accelerator electrodes of a high energy accelerator to a selected potential, such as ground or a suitable negative potential, to remove from the accelerator electrodes stray voltages that may otherwise adversely affect the ion beam.
According to one aspect of the invention, a charged particle accelerator that is operable in a high energy mode and in a low energy mode is provided. The charged particle accelerator comprises a high voltage power supply for generating a high voltage, an accelerator column coupled to the high voltage power supply and a switching assembly. The accelerator column comprises a plurality of accelerator electrodes having apertures for transport of a charged particle beam and resistors coupled between adjacent ones of the accelerator electrodes for distributing the high voltage among the accelerator electrodes. The high voltage power supply is disabled from energizing the accelerator column in the low energy mode. The switching assembly comprises one or more switching elements for electrically connecting the accelerator electrodes to a reference potential in the low energy mode and for electrically isolating the accelerator electrodes from the reference potential in the high energy mode.
In one embodiment, each of the switching elements comprises a flexible conductor having a first portion affixed to one of the accelerator electrodes and a second portion that is movable between a high energy position in electrical contact with the same accelerator electrode and a low energy position in electrical contact with an adjacent accelerator electrode. The flexible conductors may comprise conductive strips. In another embodiment, the flexible conductors comprise conductive wires formed into elongated loops. The switching assembly may further comprise an actuator for moving the flexible conductors between the high energy position and the low energy position, and an actuation rod coupled between the actuator and each of the flexible conductors.
In one embodiment, the switching assembly comprises switching elements that are respectively connected directly to the accelerator electrodes. In another embodiment, the switching assembly comprises a stack of electrically isolated conductive plates respectively connected to the accelerator electrodes, and each of the switching elements comprises a first portion affixed to one of the conductive plates and a second portion that is movable between a high energy position in electrical contact with the same conductive plate and a low energy position in electrical contact with an adjacent conductive plate.
In a further embodiment, the switching elements comprise diodes respectively coupled to the accelerator electrodes. The diodes are reverse biased in the high energy mode and are forward biased to provide a conductive path to the reference potential in the low energy mode.
In another embodiment, the one or more switching elements comprise a conductive switching bar that is laterally movable between a low energy position in electrical contact with the accelerator electrodes and a high energy position spaced from the accelerator electrodes.
According to another aspect of the invention, a method is provided for operating a charged particle accelerator in a low energy mode. The charged particle accelerator comprises a high voltage power supply for generating a high voltage and an accelerator column coupled to the high voltage power supply. The accelerator column comprises a plurality of accelerator electrodes having apertures for transport of a charged particle beam and resistors coupled between adjacent ones of the accelerator electrodes for distributing the high voltage among the accelerator electrodes. The method comprises the steps of disabling the high voltage power supply from energizing the accelerator column in the low energy mode, and electrically connecting the accelerator electrodes to a reference potential in the low energy mode.